Generally, wide angle optical lenses used in compact digital cameras are prone to significant vignetting or shading. Shading originates from the fact that light is obliquely incident on photosensitive elements located away from the center of the image sensor array, whereas light is substantially perpendicularly incident on photo-sensitive elements located at or near the center of the image sensor. In this example, the obliquely incident light may not completely fall on the remote photosensitive elements and/or may be shaded from the remote photosensitive elements by other structures of the image sensor. To obtain a more accurate representation of an object, the outputs of the pixels at different locations on the image sensor array are processed to compensate for shading effects, which vary across the image sensor array.
Conventionally, anti-shading (AS) algorithms—also referred to as shading correction algorithms—are used to compensate for this shading phenomenon. AS algorithms are based on shading models, which simulate the effects of light shining on a surface.
Conventional shading models assume simple multiplicative dependency between an ideal pixel response (e.g., when detected light is substantially perpendicularly incident upon the central photo-sensitive elements) and a shaded pixel response (e.g., when light incident on the remote photo-sensitive elements does not completely fall on the remote photosensitive elements and/or is shaded from the remote photosensitive elements by other structures of the image sensor). The corrected signal is compensated by multiplying a measured shaded raw signal by some AS gain.
Generally, the coefficient of proportionality between the shaded pixel response and the ideal pixel response is a function of the spatial coordinates of the pixel, the pixel color and the illumination type of the incident light.
A conventional model is commonly applied in connection with the Bayer color filter array. As is known, the Bayer color filter array has 4 color channels: red (R), blue (B), green-red (Gr) and green-blue (Gb). The green-red pixels are green pixels that share the same row with red pixels, whereas the green-blue pixels are green pixels that share the same row as blue pixels.
There are also 7 or 8 basic illumination types. These basic illumination types include: 2 hot or incandescent illumination types (Horizon and warm white (WW)), 5 fluorescent illumination types (IncA, cool white (CW), daylight 5000 k (DL50), daylight 6500 k (DL65) and daylight 7500 k (DL75)) and 1 optional outdoor illumination type. For convenience, the phrase “color correlated temperature” or “CCT” is used instead of illumination type. CCT is given in Kelvins and refers to the particular color of white light; the higher the CCT, the bluer the light appears.
The main target of a conventional AS model is formulated as a function of 4 three-dimensional (3D) gain functions Gcolor(Ct, x, y), which have been multiplied by respective raw pixel values resulting in images with reduced shading effects. In this example, the subscript “color” refers to one of the four color channels: R, B, Gr and Gb. Moreover, Ct refers to the CCT value of the light incident on a given pixel or pixels, and falls within a given range (e.g., [2000K:9000K]). The coordinates (x, y) are the spatial coordinates of the input pixel in the image sensor array. In conventional AS schemes, however, not all components of the shading model are CCT dependent.